Air Space in Composting Research: A Literature Review

Free air space (FAS), as a representation of the available air filled voids in a composting matrix, is considered an important parameter to define optimum substrate conditions for gas transfer. However, this parameter has been determined, interpreted, and employed in several ways by researchers. On this basis, the objective of this paper is to review the previous work related to the application of FAS in composting, providing information about the FAS concept and its importance for composting research. Also, relevant details of the methods currently used for FAS measurement and estimation are discussed. Other aspects of this review are FAS management in composting and its implications for physical and numerical models. In this way, the presented discussion provides a basis to understand the FAS concept and its potential application to future research.